HR: 12:05h
AN: T42A-08    [Abstracts]
TI: Strain Accumulation and Strain Partitioning in the Western Aleutian Subduction Zone
AU: * Steblov, G M
EM: steblov@gps.gsras.ru
AF: RDAAC / GS RAS, 6/3 Universitetsky Prospect, Moscow, 117333 Russian Federation
AU: Kogan, M G
EM: kogan@ldeo.columbia.edu
AF: Columbia University, LDEO 61 Route 9W, Palisades, NY 10964 United States
AB: The 2,200-km Aleutian megathrust demonstrates a rapid change in the sense of relative motion of the Pacific and North American plates, from nearly trench normal at Alaska to nearly trench parallel at Kamchatka, where the Aleutian and Kamchatka megathrusts connect making a cusp. This change is accompanied by an increase in the relative plate velocity from 61 to 76 mm/yr (an estimate based on our global GPS solution). Earthquake slip vector azimuths for thrust earthquakes along the arc support an idea of strain partitioning, i.e., the motion in seismic ruptures tends to be less oblique than the plate motion [MacCaffrey, 1992]. The strain partitioning predicts a steady westward motion of slivers of the hanging wall along strike-slip faults, resulting in an active collision of the far western arc with Kamchatka [Geist and Scholl, 1994]. GPS velocities measured on Aleutian islands progressively increase from 12 mm/yr at the eastern end of the arc (Kodiak) to as much as 49 mm/yr at the western end (Bering) with respect to the North American plate, which is 2/3 of the relative plate velocity. We show, using the constrained nonlinear inversion, that the high GPS velocity of Bering Is. can be alternatively explained by elastic strain accumulation resulting from locking at the subduction interface. In this scenario, there is no steady westward drift of arc slivers since the elastic strain is periodically released in earthquakes, with the islands returning to their original positions. Geodetic observations lasting for about a decade (continuous GPS was installed on Bering Is. in 1996) do not allow us to discriminate the periodic elastic strain accumulation from the along arc steady strike-slip motion. Yet there is a good argument in favor of the elastic strain at the interface beneath Bering Is.: small GPS velocities in Kamchatka at the Aleutian-Kamchatka cusp (<14 mm/yr) are easily explained by superposed elastic strains at Kamchatka and Aleutian subduction interfaces. There is simply no evidence in GPS velocities of intense compression near Cape Kamchatka as predicted by the collision scenario. Moreover, specific azimuths and values of GPS velocities in Kamchatka at the cusp can be easily explained by superposed elastic strains. Some amount of slivering in the westernmost Aleutians is clearly evidenced by abundant strike-slip seismic focal mechanisms, yet the rate of motion along fracture zones to the north and to the south of the arc currently is unknown; it can be small with respect to the elastic strain accumulation in an azimuth parallel to the arc. Unless longer time series at larger number of islands are observed, a unique interpretation of GPS velocities on the Aleutians is not possible. Recently, the first epoch of GPS at Medny (Copper) Is. was carried out, in 100 km from Bering and at greater distance from the Aleutian Trench. Comparison of GPS velocities at Bering and Medny should provide evidence on how significant is the strain partitioning. Bürgmann, R., M.G. Kogan, G.M. Steblov, G. Hilley, V.E. Levin, and T. Apel, Interseismic Coupling and Asperity Distribution Along the Kamchatka Subduction Zone, J. Geophys. Res., 110, B07405, doi:10.1029/2005JB003648, 2005.
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 7299 General or miscellaneous
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005